194 THE BOOK OF NATURE STUDY 



the earliest vertebrates are fishes ; these are followed later by 

 amphibia and reptiles ; and these by birds and mammals. In 

 each division of the animal kingdom there is progress from lower 

 to higher forms, a thing which may be especially recognised 

 in the brachiopoda, the Crustacea, the fishes, the birds, and 

 the mammals. Thus it becomes possible to determine from the 

 status of the highest organisms present in any particular stratum, 

 and from the general grade of the fossils, each in its own division 

 of the plant or animal kingdom, the approximate position of 

 the stratum in the geological scale. Progress of this kind has 

 occurred, not at the same rate or absolutely in the same direction, 

 all over the world. But over the open oceans, always occupying 

 a larger area than the land masses, and always more largely 

 represented in the strata than fresh water or land deposits, the 

 progress has been more uniform. Thus correlation even over 

 great distances becomes a possibility. 



Historical nomenclature founded on the progress of life is 

 therefore the type which will be the most exact, the most minute, 

 the widest in its application, and every effort is being made 

 in geology to obtain in this way a time-scale which shall be of 

 world-wide application. Towards such a system words like 

 Eocene and Miocene, Paradoxidian Division, Pentamerus Beds, 

 are contributions, but the tendency at present is to attach to 

 the place-names an exact biological significance, and to continue 

 the use of them in this new sense. Thus the word Silurian attains 

 a new significance, and many other words, such as Valentian, 

 Bathonian, Cenomanien, Ypresian, have been coined from 

 place-names to serve the same purpose. 



Apart from their use in giving us the most convenient and 

 widely applicable way of dividing the entire record into sections 

 and chapters, fossils are capable of extended use as date indexes 

 of wonderful exactness, and as means of minute correlation. 

 It is found that the fossils of a stratum include usually four 

 types in varying proportion : (i) Those confined to the bed 

 itself and not found above or below ; (2) those found also in beds 

 below ; (3) those found also in the beds above ; (4) those found both 

 above and below. All four types are useful in enabling an estimate 

 to be formed of the age of the rock containing them, but those 



